The R-T-B based permanent magnet (R is rare earth element(s), T is Fe or Fe with a part of which replaced by Co, and B is boron) having a compound composed of a tetragonal R2T14B structure as the main phase is known to have excellent magnetic properties and has been a representative permanent magnet with high performance since the invention (Patent Document 1) in 1982.
The excellent magnetic properties of the R-T-B based permanent magnet are attributable to magneto crystalline anisotropy of the tetragonal R2T14B structure. Especially, the R-T-B based permanent magnets with the rare earth element(s) R being consisted of Nd, Pr, Dy, Ho and Tb have a large magneto crystalline anisotropy and are preferably used as permanent magnet materials. However, the rare earth element(s) R is unevenly distributed in some regions, and thus anxiety is caused from the viewpoint of the supply stability. In addition, as the rare earth element(s) R is easy to be oxidized, its corrosion resistance is low and an action such as Ni plating on the R-T-B based permanent magnets or the like is needed to prevent the rare earth element from oxidizing.
The magneto crystalline anisotropy is determined by the atomic arrangement and the shape of the electron cloud in the crystalline structure. In the mentioned compound composed of the R2T14B structure, the c axis is parallel to an axis of easy magnetization, which is caused by that the R2T14B structure is tetragonal. That is, that the crystalline structure is not isotropic is required for an excellent permanent magnet material and it is essential to show a high magnetic anisotropy.
Meanwhile, FeCo (i.e., permendur) is known as a practical material having a high saturation magnetization Is. The saturation magnetization Is of FeCo is 2.4T. It is much larger than the saturation magnetization Is (i.e., 2.2T) of Fe (i.e., pure iron), not to speak of the saturation magnetization Is (i.e., 1.6T) of Nd2Fe14B which is a representative R-T-B based permanent magnet. However, FeCo has a bcc (body-centered-cubic) structure and an isotropic crystalline structure. Thus, its magnetic anisotropy is low and it can be used as an excellent soft magnetic material. That is, it is not suitable as a permanent magnet.
However, Non-Patent Document 1 suggests that the crystalline structure will become anisotropic and the magnetic anisotropy will be shown by distorting the crystalline structure of FeCo which has the isotropic body-centered-cubic structure to be a body-centered tetragonal structure. If magnetic anisotropy which is the source of the coercivity Hc is shown in FeCo, combining with its high saturation magnetization Is, it can be expected as an excellent permanent magnet material. However, the suggestion is based on first-principles calculation simulation, and thus the calculated value loses touch with the practical physical properties, for example, the calculated value is the value under absolute zero (0K).
According to the calculation simulation, the required tetragonal distortion for showing a sufficient magnetic anisotropy in FeCo is very large. It is required that the ratio of c/a of the c axis (which is the drawing direction) and the a axis (which is the compression direction) is about 1.2. The value is greatly larger than the elastic limit of the metal. Even a distortion to is applied to FeCo to reach c/a=1.2, distortion will not be introduced due to the plastic deformation which is caused by interatomic slip. That is, it is believed that it is very difficult to obtain a permanent magnet using the so-called FeCo alloy.    Patent Document 1: JPS59-46008    Non-Patent Document 1: Physical Review Letters, 027203-1, Volume 93, Number 2 (2004) “Giant Magnetic Anisotropy in Tetragonal FeCo Alloys”    Non-Patent Document 2: Digests of the 38th annual conference on magnetics in Japan, 4aE-1 (2014) “Magnetic properties of Rh/FeCo film grown on MgO (001) substrate”